This invention relates generally to complementary field effect transistors (CMOS) and complementary modulation doped field effect transistors (CMOD), and more particularly the invention relates to such transistors having structures which enhance and equalize current carrier mobilities with a built-in SiGe quantum well.
There is presently much interest in the use of Ge.sub.x Si.sub.l-x alloy strained layers in microelectronic applications. See for example Bean et al. U.S. Pat. No. 4,879,256 having a strained superlattice semiconducting system which can be used to produce order-disorder phase transitions in semiconductor alloys; Bean et al. U.S. Pat. No. 4,661,829 having germanium-silicon alloys with strained superlattice structures; and Dresner U.S. Pat. No. 4,697,197 having alternate interleaved layers of materials in a superlattice channel region. The compatibility of GeSi alloys with silicon based technology provides the convenient implementation of any development and advancement. Recently, heterojunction bipolar transistors using Ge.sub.x Si.sub.l-x /Si strained layers have demonstrated potential applications in high speed computers, communication and signal processing. See Plummer and Taft U.S. Pat. No. 4,825,269. Cut-off frequency, an indicator for the performance, above 75 GHz has been recently reported. In addition to the intrinsic speed and frequency advantages, the presence of a proper strain offers yet higher carrier mobility and can further enhance the device and circuit performance. Moreover, the mobility of the layers can be increased if the dopants are placed in the barrier region of the heterojunction.
In Al.sub.x Ga.sub.l-x As./GaAs heterojunction structures, such art of increasing mobility has been reported and implemented in so called high electron mobility transistors (HEMT) or modulation doped field effect transistors (MODFET). Similarly, Ge.sub.x Si.sub.l-x /Si MODFETS have been demonstrated.
Recently there has been a thrust in integrating conventional homojunction bipolar and CMOS devices on the same semiconductor chip since the overall performance benefits can be derived from both of the devices. This silicon technology is known as BiCMOS. The drawback of the conventional CMOS is the large difference of the electron and hole mobilities. For MOS, the channel electron mobility is about 700 cm.sup.2 /V.sub.s and the channel hole mobility is substantially below the electron mobility.